Welcome back!
I will list up the points of the commenters above and add some hints. You will find most of these infos described at the Phoenix wiki, read them

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Phoenix Eggs, which drop while you lvl solo, can be turned in at your trainer (or master trainer) for extra XP.
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Xp Items drop at certain mobs and can be turned in at the related NPC (right click the xp item for Infos) for extra XP.
Roughly ranging from about 7bubs at low lvl to 4 bubbs at lvl 49.
You can only turn in 10 items per lvl, 10 items of the same type per character and if you turn in the items and lvl up during the process some items of the stack count for the next lvl. So only use them one by one if you are close to a level up in order to track your turn in limit for that level.

You should ALWAYS try to level at spots where xp items drop until you have 10 of them. You can also buy them at the housing zone, better ask your guild for spare xp items.
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Kill Tasks provide you bonus XP for killing a certain amount of mobs of the same type / in RvR zone / other criteria. You can see your progress at /task .
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Mob Type Bonuses are provided for mob types that get rarely killed. In some cases up to 300+% xp. One of the /task commands will show you which types give you an XP bonus.
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RvR Tasks, available at lvl35+ also give you a certain amount of XP (RPs too). In some cases all you need to do is die in a RvR zone in order to cash up to 3/4 bub every 45 minutes. Even more if you additionaly participate in the
Keep Raid Tasks. Some people solely level while running with the Zerg after lvl 40+.
I recommend to do a RvR run everytime you leave your level spot anyway (ideally every 45 minutes) for those sweet extra XPs and RPs.
Alltogether leveling is damn "easy" and rewarding compared to Uthgard or classic live servers, but there is one thing I need to remark!
DO NOT create a stealther as your first character. Leveling is not that kind of a problem but farming the ressources for a proper SC/Armor IS!
In order to get suited for RvR you will need to farm a certain amount of platin / ROG's (randomly generated items) and phoenix feathers (custom currency used to buy items usually dropped by epic mobs). The amount of those ressources needed to create a template for a caster is easily farmed in a short period of time.
Those needed for a proper stealther template can be a chore (if your only farming character is a Stealther).
With the proper support of a friendly guild this may not be the case, but that is nothing I would entirely rely on.
Therefore I recommend to create a caster class suitable for farming first.
Check out the related Realm Sub-Forum for some suggestions but usually considered the best cloth wearing solo farming classes are: Animist, Necromancer or Animist.
Have fun and enjoy the nostalgic moments combined with great qol changes.